Structure−property relationships of the 10H Hexagonal-Type Perovskite BaMn0.4Fe0.6O2.73

The crystal structure of BaMn0.4Fe0.6O2.73, a recently identified oxygen-deficient 10H-type hexagonal perovskite in the system BaMn1-xFexO3-y, has been established by a combination of electron microdiffraction and neutron diffraction. The structure (space group P63/mmc, a = 5.74435(5) Å, and c = 24.0331(3) Å) can be described by a stacking sequence (hch‘ch)2 along the c-axis with the anion deficiency located exclusively in the h‘-BaO3-type layers. The anion distribution in the h‘-BaO3 layers differs significantly from that observed for 10H BaFeO2.8 and results in a 70:30 random distribution of corner-sharing tetrahedral Fe2O7 dimers and face-sharing octahedral (Mn,Fe)2O9 dimers as opposed to the exclusive Fe2O7 dimers in 10H BaFeO2.8. The difference is attributed to the preference of Mn for octahedral coordination. The compound is a “leaky” insulator at room temperature with a permittivity of 20. The conduction mechanism has low activation energy, 0.3 eV, and is consistent with polaronic hopping associated with the Fe and/or Mn ions.